1. Field of the Invention
This invention relates to a system for controlling the air/fuel ratio of an internal combustion engine. More particularly, it relates to a system for controlling the air/fuel ratio of a multicylinder internal combustion engine in which the air/fuel ratio to be applied to the engine is intentionally perturbed or oscillated between lean and rich directions in order to enhance the purification efficiency of a catalytic converter installed at the engine's exhaust system. This is known as the perturbation effect.
2. Description of the Prior Art
The perturbation effect is often described in papers and has been a known technique, as well as the phenomenon of the catalytic converter's storage of oxygen in order to achieve the optimum purification efficiency of the catalytic converter. The catalytic converter's oxygen storage is a phenomenon in which the catalytic converter stores oxygen when the air-fuel mixture is rich and discharges the same when the air-fuel mixture is lean. The perturbation effect is explained in Japanese Laid-Open Patent Publication No. Sho 64(1989)-56,935, for example. In the prior art technique disclosed in that publication, a desired air/fuel ratio is forcibly oscillated or perturbed between the rich and lean directions, centered on the stoichiometric at a cycle (frequency) and an amplitude determined with respect to engine speed and engine load.
In the prior art technique, however, when the engine operating condition varies continually, the desired air/fuel ratio is fixed either at the lean or rich side. It therefore becomes impossible to attain the purpose of the perturbation control sufficiently to improve the purification efficiency of the catalytic converter.
An object of the invention is therefore to overcome the problem and to provide a system for controlling the air/fuel ratio of an internal combustion engine in which a desired air/fuel ratio at a predetermined cycle and amplitude is supplied to the engine irrespective of whether or not the engine is in a steady-state operating condition or a transient operating condition--in other words irrespective of the change in speed or load of the engine--so as to sufficiently enhance the purification efficiency of the catalytic converter.
Further, in the prior art technique disclosed in the publication, a single air/fuel ratio sensor is installed at a confluence point of the exhaust system of a multicylinder internal combustion engine to detect the air/fuel ratio of the mixture supplied to the engine, and the air/fuel ratio is feedback controlled to a desired value such that the error therebetween is decreased. However, the exhaust gas at the confluence point is a mixture of the exhaust gases evolved from the individual cylinders and therefore does not indicate respective air/fuel ratios at the individual cylinders. In other words, in the prior art technique, the perturbation control is not conducted separately for the individual cylinders of the engine.
A second object of the invention is to provide a system for controlling the air/fuel ratio of a multi-cylinder internal combustion engine in which the air/fuel ratio is controlled separately for the individual cylinders to conduct the perturbation more effectively, thus further improve the purification efficiency of the catalytic converter.
In the prior art technique, furthermore, the deviation between the desired air/fuel ratio and the detected air/fuel ratio is multiplied by a gain to yield a feed-back correction factor. As a result, it becomes impossible successfully to carry out the perturbation control at an engine operating condition in which air/fuel ratio control is conducted in an open-loop fashion.
A third object of the invention is therefore to provide a system for controlling an air/fuel ratio of an internal combustion engine in which the perturbation control can successfully be carried out even at an engine operating condition in which air/fuel ratio control is conducted in an open-loop fashion.
For realizing the objects, the present invention provides a system for controlling an air/fuel ratio of a multicylinder internal combustion engine such that an actual air/fuel ratio at, at least one of upstream and down-stream of a catalytic converter installed at an exhaust system of the engine, is intentionally oscillated in at least one of its amplitude and cycle. The system comprises first means for establishing a characteristic of a desired air/fuel ratio as a periodic function such that the desired air/fuel ratio varies at at least one of a predetermined amplitude and cycle within a predetermined period, second means for sampling the characteristic by a time interval determined on the basis of a time interval between TDC crank angle positions of the engine, third means for determining each cylinder's desired air/fuel ratio from the sampled data, fourth means for determining a fuel injection amount for each cylinder from each determined cylinder's desired air/fuel ratio, and fifth means for supplying a fuel to each cylinder in response to the determined fuel injection amount.